1. Field of the Invention
Example embodiments of the present invention relate generally to a receiver and method thereof, and more particularly to a receiver for over-sampling a received signal and method thereof.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) system may be used to modulate digital signals in accordance with any conventional digital communication standard, such as xDSL, IEEE 802.11, IEEE 802.16, etc. Conventional digital communication standards may use a given number of sub-carrier waves and frequency bands. However, generally, each of the conventional digital communication standards may employ a similar modulation scheme.
In conventional digital modulation, an Inverse Fast Fourier Transform (IFFT)/Fast Fourier Transform (FFT) function block may modulate and demodulate data, respectively. A cyclic prefix may be inserted in a transmitted signal to compensate for impulse response characteristics and/or multi-path fading in a channel so as to reduce intersymbol interference (ISI) and/or inter-channel interference (ICI). Thus, if the impulse response characteristics or delay in a channel is shorter than the cyclic prefix, the ISI may be generated within a given range of the cyclic prefix. Such an ISI may be reduced by removing or reducing the cyclic prefix. However, if the ISI is longer than the range of the cyclic prefix, the ISI and/or the ICI may not be mitigated before a time domain equalization method may be employed, for example, in an asymmetric digital subscriber line (ADSL).
Additive White Gaussian Noise (AWGN) may be included in a channel. An analog low-pass filter (LPF) may be used to suppress the AWGN in the channel. The analog LPF may not be able to completely remove the AWGN from the channel. Thus, a digital filter may be employed for further reducing the AWGN (e.g., portions of the AWGN not removed by the analog LPF or generated after the analog LPF reduces the AWGN). The analog LPF may also be used to reduce an aliasing effect by sampling the multi-carrier wave signal.
A conventional digital filter may not reduce or remove noise in a pass-band of the analog LPF. Furthermore, the conventional digital filter may include a number of delay taps to reduce noise. The delay taps may degrade the impulse response of a channel. Thus, a tradeoff between impulse response degradation and noise reduction may be made by selecting the number of delay taps in the digital filter.
FIG. 1 is a block diagram illustrating a conventional OFDM receiver 100. Referring to FIG. 1, OFDM receiver 100 may include an analog-to-digital converter (ADC) 10, a digital filter 20 and a FFT calculating unit 30 for demodulating an OFDM signal received through a multi-carrier wave signal.
Referring to FIG. 1, an OFDM receiving signal may be an AWGN reduced (e.g., removed) signal. In other words, an input signal of a system may be converted into a baseband signal and the AWGN may be removed/reduced from the baseband signal by the low-pass filter to generate the OFDM receiving signal. The OFDM receiving signal may further be an analog signal after an aliasing effect by the multi-carrier wave may be reduced from the input signal.
Referring to FIG. 1, The ADC 10 may convert the OFDM receiving signal from an analog format into a digital format by sampling the OFDM receiving signal in a given sample period. The given sample period may be a period of a Nyquist frequency.
Referring to FIG. 1, The digital filter 20 may be a digital base-band filter for filtering digital symbols outputted from the ADC 10. The digital filter 20 may reduce noise not previously removed by the analog filter and/or AWGN generated after the analog-to-digital conversion performed by the ADC 10. The digital filter 20 may further reduce quantization noises.
Referring to FIG. 1, the FFT calculating unit 30 may perform an N-point decimation FFT on each of the digital symbols output from the digital filter 20. Thus, the FFT calculating unit 30 may demodulate digital data, where the digital data may be previously modulated by performing an IFFT at a transmitter.
As described above, the OFDM receiver 100 may reduce the AWGN by using the analog low-pass filter and the digital filter in combination so as to improve a signal-to-noise ratio (SNR) of FFT calculation. However, the AWGN may not be fully removed by the above-described conventional process. Furthermore, the digital filter may include a number of delay taps correlating to a level of noise reduction, thereby increasing noise reduction at the expense of degrading an impulse response of a channel.